Using ERASER to remove background

I am just taking my first steps in Gimp and have been looking at a number of on-line tutorials. I have chosen to use the eraser with fuzzy edge to remove the background from a dog subject. I created an Alpha channel and made a copy of the original image as suggested in the tute. When I begin to erase it works for a few stroke then stops erasing. By trial and error I found that If I select back and forth between the original and the copy it will erase a little on each but this seems to be an irregular way of doing the job and this is not what the tutorials show. What I erase shows up on both the original and the copy, that is they compound the erasing. Should this occur? Why can't I complete the erasing on just one image. While it does achieve the removal of the background I feel I am doing something wrong. Can you make any suggestions? Many thanks for reading this.